Fix Vim indent regressions from #13600.

The change in #13600 was incorrect, containing a bad regular expression;
inside an indent function, errors are silently ignored (and the ``~=``
operation will return 0), so it just always failed, causing the cases
that were supposed to be caught to not be caught and making things like
the ``match`` example shown above or struct field definitions regress.

I have fixed the regular expression to what it should have been. This is
still imperfect, of course, not handling cases like where the first
argument to a function is a function call (``foo(bar(),``), but it'll do
for now.

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I have a general request to make of reviewers about any changes made to `src/etc/vim`: **please tell me**. As a general rule I want to review them. (I’ll make an exception for changes the prelude; it needs fixing from time to time when some people don’t update the syntax file, anyway.)

cc @brandonw